We report on Terahertz (THz) detectors based on III-V high-electron-mobility field-effect transistors (FET). The detection results from a rectification process that is still highly efficient far above frequencies where the transistor provides gain. Several detector layouts have been optimized for specific applications at room temperature: we show a broadband detector layout, where the rectifying FET is coupled to a broadband logarithmic-periodic antenna. Another layout is optimized for mixing of two orthogonal THz beams at 370 GHz or, alternatively, 570 GHz. A third version uses a large array of FETs with very low access resistance allowing for detection of very short high-power THz pulses. We reached a time resolution of 20 ps.
We report on arrays of THz-emitters based on n-i-pn-i-p-superlattice photomixers for imaging and spectroscopic
applications. The output power of a n-i-pn-i-p superlattice photomixers recently has reached nearly 1 μW at 1 THz with
a broadband antenna. There are no fundamental physical limitations at this stage for further improvement. Tunable
continuous wave (CW) THz-sources for imaging and spectroscopy are highly desired tools for security and
environmental applications. In particular, most stand-off imaging applications require a rather high THz power to allow
for a sufficient dynamic range, and a narrow illumination spot size for high spatial resolution. Both goals can be reached
by using an array of mutually coherent photomixers. We have simulated beam patterns for an arbitrary number of
mutually coherent single sources with respect to a small beam size and high peak intensity. Here, we confirm the
simulations experimentally by an array of 4 sources with a 4 inch THz optics. The beam profile is measured in the target
plane at a stand-off distance of 4.2 m. As a result, the beam diameter is reduced by a factor of 6 and the peak intensity is
enhanced by a factor of close to (4)<sup>2</sup> = 16, in excellent agreement with our simulations. Such an arrangement allows not
only for high resolution stand-off imaging but also for spectroscopic investigations at stand-off distances. The THz
frequency can be tuned over more than a decade (i.e. 0.1 to 2.5 THz) by tuning the wavelength of the mixing lasers. The
spectral linewidth of the THz sources is only limited by the linewidths of the mixing lasers and can be made extremely
narrow. A straightforward demonstration is achieved by water vapor spectroscopy in laboratory air with a single source.
We report on simulations of arrays of free standing THz sources for high brightness applications in THz active imaging and sensing. THz-photomixing sources for wide tunable, room-temperature, narrow linewidth, and CW operation are considered. All the source elements are coherently driven to allow for controlled interference of the beam pattern. The center peak not only gains more power but also becomes much narrower due to interference, compared to a single emitter. The peak intensity increases with the square of the number of sources. This can improve both the resolution and the dynamic range for stand-off active imaging and sensing applications. We discuss the effect of different source layouts with respect to the illumination pattern on the target.
CW-photomixing semiconductor devices have hardly exceeded an output power of 10 μW around 1 THz.
Availability of a few mW, however, would stimulate the demand for THz-imaging, -scanning, and spectroscopy.
Increasing the poor power conversion efficiency from the optical pump to THz-output is most
desirable. On the other hand, the thermal threshold "per pixel" is limited to about 100 mW of pump laser
power. So both limits have to be pushed towards higher performance. In this paper
we report on arrays of photomixing devices to overcome the thermal threshold limit.
If each individual photomixer in the array can be driven to the same thermal threshold power,
the overall THz output can be larger by a factor N\times M for an array. The power of directed emission,
however, can be increased even by a factor (N x M)<sup>2</sup> compared to the individual device.
In addition, by adjusting the two laser beams slightly noncollinear, a directional control of the
emitted THz-beam is achieved. The angular difference of the incident beams is
enhanced by the ratio of the THz-wavelength (≈300 μm) and the optical wavelength (≈0.85 μm)
with regard to direction of the emitted THz-beam. Thus, a full steering of the THz beam
can be achieved by tuning this angle by less than 1 degree (17.5 mrad).